We and our Tokyo colleagues have now measured 53Mn in over one hundred Antarctic meteorites, including 88 ordinary chondrites [1,3,5,6 and unpublished data]. Table 1 shows results obtained since our earlier summary [7]. These include both Yamato and Allan Hills objects. In all cases shown, at least...
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.218.144 http://www.lpi.usra.edu/meetings/lpsc1981/pdf/1267.pdf |
| Summary: | We and our Tokyo colleagues have now measured 53Mn in over one hundred Antarctic meteorites, including 88 ordinary chondrites [1,3,5,6 and unpublished data]. Table 1 shows results obtained since our earlier summary [7]. These include both Yamato and Allan Hills objects. In all cases shown, at least one other cosmogenic nuclide has been measured, usually on the same or an adjacent sample. This program has had several objectives. Our first interest was in the determination of terrestrial age. AS Fig. lmakes clear, this has proven in general, to be much less than the 3.7~10 year half-life of 53~n. From other measurements it seems to be typically on the order of one to a few hundred thousand years. Second, the discovery of low 53Mn values associated with high content of other nuclides, best explained by two- or multi-stage bombardment histories, suggested the importance of a statistical study of such objects [8]. Finally, at a practical level, cosrnogenic nuclide patterns are of great value in identifying stones which form part of a single shower. The following new results seem to be of special interest: (1) The objects Y74028 and Y74116 were found 2.5 lan apart. Their low 53Mn and 5-8x10 ~ yr 4 0 ages ~ suggest strongly a two-stage bombardment history. It seems likely that they are part of the same fall, though more detailed classification is needed to confirm this. If so, they represent a third case, along with Y7301 and ALHA 76008 [I]. A simple two-stage model yields a duration for the second stage of bombardment <9x10 5 yr for the new pair. Measurement of shorter-lived radionuclides is needed. Another possible candidate is Y74094(H6), with 31 dpm53~n/kg(~e+l/3~i), for which we have no other isotopic data. Even so, demonstrable cases of twostage bombardment are now only a few percent of the total, much less than Wetherill [8] suggests. Perhaps we need more detailed model predictions of radionuclide patterns. (2) There are several Yamato objects with apparent very long 21~e ages: |
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